The present invention relates in general to pressure transducers, and pertains more particularly to liquid-filled pressure transducers, such as pressure transducers of the type disclosed in U.S. Pat. No. 3,349,623 to Pastan issued Oct. 31, 1967 and U.S. Pat. No. 3,678,753 issued to Eggleston, et al July 25, 1972. Even more particularly, the present invention relates to relatively lower range melt pressure transducers.
As discussed in both the Pastan patent and the Eggleston, et al patent, fluid-filled pressure transducers are designed to be used in those systems in which it is undesirable for the medium whose pressure is being measured to enter into the instrument. In such cases, the instrument itself is filled with a fluid which is coupled by means of a diaphragm or some other device to the medium whose pressure is to be measured. The fluid which fills the instrument directly transmits the pressure of the medium to the sensing device. A typical fluid for the transducer is mercury.
The Eggleston, et al U.S. Pat. No. 3,678,753 described a pressure transducer that includes an elongated frame and a capillary tube extending through the frame and terminating at one end adjacent one end of the frame. A coupler closes that end of the frame and defines with the frame a chamber that is in communication with the capillary tube. The other end of the capillary tube communicates with a sensing device which has a small deflection throughout its full operative range. The sensing device is in the form of a deformable cap member having a recess therein, defining with a portion of the frame, a thin sensor compartment that is of disc-shape and is in communication with the other end of the capillary tube. A liquid, preferably mercury, fills the thin sensor compartment, chamber and capillary tube so as to transmit directly the pressure applied against the coupler to the sensing device to render a pressure measurement.
The Eggleston, et al pressure transducer is generally designed, however, for pressures down to about 1500 p.s.i. With this prior art transducer, pressures lower than 1500 p.s.i. are not effectively measured because the deflectable wall or diaphragm of the cap member has to be constructed too thin making the device quite susceptible to manufacturing and fracture problems.
Accordingly, it is an object of the present invention to provide an improved fluid-filled pressure transducer that is in particular adapted for making low pressure measurements such as those in the range of 250-1500 p.s.i. More particularly, the present invention carries out this low pressure measurement without requiring a large component design. In particular, the present invention permits low pressure measurements using the standard 1/2 inch - 20 threaded snout.
Another object of the present invention is to provide an improved low-range melt pressure transducer that is relatively simple in construction and that can be fabricated relatively inexpensively.
Still another object of the present invention is to provide an improved technique for constructing the sensing device.